In the semi-conductor field, and with particular reference to semi-conductor devices such as diodes and the like, including the silicon crystalline form of a wafer and electrical conductors connected to opposite sides of the wafer, it has been customary to employ glass for the encapsulation of such objects to protect them. The encapsulating glass is hermetically sealed to the electrical conductors to guard against harmful impurities including water vapor from the atmosphere. It has also been the practice to employ certain glass compositions for drawing into various shaped glass articles, as well as for sealing purposes. Various glass compositions have been devised in the past which have been satisfactory for the production of small glass containers. However, it has been observed that certain properties of these glasses have interfered with obtaining satisfactory sealing of these devices and, moreover, have deleteriously affected certain delicate electronic parts which have been packaged or sealed therein. An example of a deleterious effect is the so-called "poisoning" effect which glasses containing an alkali metal oxide have on electronic parts, and which prohibits the use of such glasses for packaging and sealing of these components.
In the sealing of various metal surfaces, it is of considerable importance that the sealing glass possess sufficient stability to resist uncontrolled devitrification in the course of sealing or during subsequent treatments of the sealed metal part. Undesirable devitrification may cause loss of seal strength and impart other complications leading to the deterioration of an adequate seal. Also, once the metal has been sealed, the glass should be resistant to chemical attack which may result from exposure to solvents, effect of water or other aqueous solutions or the general atmosphere.
Another important requisite in obtaining good sealing strength is that the sealing glasses possess a coefficient of thermal expansion that approximates as closely as possible that of the metal to which it is sealed. If the rates of thermal expansion are significantly different, a satisfactory bond may not be maintained upon cooling of the seal when first made. Even if initially satisfactory, the seal between the glass and the metal may fail later as a result of temperature fluctuations. Thus, in the past it has been particularly difficult to form adequate seals to molybdenum, alloys of molybdenum, iron-nickel-cobalt (KOVAR) alloys and similar alloys because many sealing glasses do not have the proper coefficient of thermal expansion. Consequently, there has developed in the art a need for sealing glasses which overcome the aforementioned disadvantages.